A wave source moves at a constant speed \(v_s\) through a stationary medium toward a stationary detector. The speed of the waves relative to the medium is \(v\), where \(v_s < v\). The source emits waves with a constant period \(T_0\) and unshifted wavelength \(\lambda_0\). Which of the following expressions correctly represents the apparent wavelength \(\lambda'\) measured by the detector directly in front of the moving source?